AMONG THE MANY "what ifs" that plague our memories of Vietnam is the "what if" we had known the Vietnamese better -- their culture, religions, politics, economy and sociology. Had we known more, would we have responded differently in war or in seeking peace and the extrication of our troops?

About the only potential source of informed knowledge of Vietnam available to policy makers in the 1960s was the French experience, laid out in such works as the classic research of Pierre Gourou, whose Les Paysans du delta tonkinois: Etude de geographie humaine was published in 1936. Many French administrators and academics were steeped in the experience of French colonization, from 1867 when the French took control of Cochinchina (the Meking Delta portion of today's Vietnam) until their withdrawal in 1954 after the Dien Bien Phy debacle. These students of Vietnam recorded invaluable data and impressions that were denied us in the '60s by sour U.S.-French relations and weak U.S.-French acedemic links. There was no conduit among faculties of human geography, history and economics that allowed U.S. policy makers to fill a critical information gap. Besides, what could we learn from the French anyway? And from the mid-'60s, of course, few U.S. or French academics were on speaking terms with U.S. policy makers concerned with Vietnam.

As a small band of young academics, troubled by the U.S. experience, were learning in the last '60s, careful review of the French experience could have helped immeasurably. One of those was Samuel Popkin, whose The Rational Peasant contains the results of a decade's research based largely on French sources.

Popkin tells us in several startling chapters in the middle of his book that there was never such a thing as Vietnam. It was even, until the mid-20th century, two distinct entities -- Cochinchina to the south, and Tonkin (the Red River Delta) in the north together with Annam (or the highlands). Village elders in these two regions played entirely sissimilar roles.Economically and administratively, village life evolved and was directed differently.

Popkin probes the cultural and social fabric of these two regions to find reasons for this. He points to anomalies such as the success of efforts to spread Catholicism to Tonkin in contrast to the failure of such efforts in Cochinchina. Buddhism overlaid ancestor worship in Cochinchina, whereas Confucianism overlaid it an Annam and Tonkin. Tonkin's economy was static and in decline, whereas in Cochinchina, an economic "frontier" was opened by French technology and medicine. Peasants starved in Tonkin and prospered in Cochinchina. Events moved so rapidly in Cochinchina that the mandarins never achieved full control as they did in the north. These examples, simplified here but studiously presented by Popkin, only illustrate the abundant evidence he has uncovered.

Unfortunately, Popkin's revelations are hidden behind two chapters, constituting almost a third of his book, that are so stuffed with arcane jargon they are nearly impossible to read. (No doubt they are designed to gain or preserve tenure. Can't a young professor get promoted for solid research alone?) For his academic colleagues, the author seeks to prove that the peasant response to colinization is best described by a "political economy" as opposed to a "moral economy" approach. For the rest of us, he is saying that those who argue -- as did Frances FitzGerald in her prize-winning Fire in the Lake -- that an idyllic peasant communal life in the Tonkin and Mekong area was disrupted by the ugly rationalism of the western colonist (in this case the French) are wrong. On this point Popkin, though lacking Fitzgerald's flourish, seems to have the compelling facts on his side. He argues that the French presence and practices actually boosted village life in many ways and that it was the Vietnamese peasant who was rational in his response.

Popkin's research raises questions about our Vietnam policy that one hopes he will pursue. As we witness the boat people's exodus from Vietnam, we must wonder if the political geography of that nation state will ever be settled by choice, rather than force. Looking back, I can see two instances when insights like Popkin's could have helped. Our support of President Ngo Dinh Diem, from 1955 until he was killed seeking sanctuary in a Catholic church after a November 1964 coup, was flawed throughout. To the Cochinchinese he was first a Catholic; second an Annamite mandarin. Consequently he was a foreigner. Yet we backed him.

The second instance occurred in 1971-72 when militarily Vietnam came unravelled from the highlands (Annam). As we withdrew our forces, the "main force" (the number of army divisions) balance there and throughout Annam tipped in favor of the North Vietnamese. In 1971 Dr. Kissinger was urged to tell President Thieu to make his stand in Cochinchina and abandon Annam. The French sources, which Popkin has mined so thoroughly and with which some of Kissinger's advisers were familiar, indicated that the shared traditions of Cochinchina and Annam-Tonkin were weaker than those of the United States and Canada. And consequently the South Vietnamese main and territorial forces could not defend Annam and Cochinchina. Of course, by then it was far too late. Thieu's support of our negotiations was deemed the pivotal factor in getting our troops out, and to Kissinger it sounded like an argument from left field.

Popkin's last chapters suggest our policies for the previous two decades should have addressed Tonkin and Annam, where revolution was inevitable, differently from Cochinchina, where evolution was certainly possible. I doubt anyone from the State Department or the CIA ever suggested these areas were different, despite nearly two decades of opportunities. As we search today in films and elsewhere to add depth to the harshness and sterility of our Vietnam experience, we must hope Popkin's work is joined by others who have his determination to understand that bitter experience.